1. Field of Invention
The present invention relates to a pneumatic radial tire. More particularly, it relates to a pneumatic radial tire having improved drainage with low noise for use in high-speed large trucks or automobiles.
2. Description of the Prior Art
In automobiles running at high speed on rainy days, rainwater penetrates and is wedged between the tread of a tire and the surface of a road to cause a phenomenon called hydroplaning in which the friction between the tread and the road surface is lost and the steering wheels and brakes do not correctly function. Thus safety of driving is compromised.
Conventionally, the pneumatic radial tire 1 as shown in FIG. 15 is proposed to solve the problem of hydroplaning in automobiles. This pneumatic radial tire 1 is formed as a toroid with rubber layer 2, and the beads 3 which are provided with a pair of bead cores 4 formed into a ring around the rotating shaft of the radial tire 1. A 2-ply carcass layer 6, consisting of radial ply cords 5 arranged with the ends of the carcass layer 6 turned inside to outside as they are wound around a pair of bead cores 4.
The radial outside of carcass layer 6 is provided with a 2-ply steel belt layer 8 consisting of substantially nonstretchable steel cord 7, and radially outside of the steel belt layer 8, a tread 9 is formed to cover the steel belt layer 8. Further, the surface of the tread 9 is provided at predetermined intervals with a plurality of continuous rib grooves circumferentially extending around the radial tire 1 to permit water entrapped between the tread 9 and the surface of a road to be discharged outside via the circumferentially extending grooves 10, thus improving the drainage of the radial tire 1.
In the radial tire 1 as proposed above, however, there occur air-column tubes between the circumferentially extending grooves 10 and the surface of a road in the rolling tire, and the air in the air-column tubes causes noise by resonating with the vibration of the radial tire 1 brought about by the uneven surface of a road. If circumferential grooves 10 are enlarged to secure drainage, the noise will increase because the air-column-tube resonance sounds are raised generally in proportion to the size of circumferential groove 10.
As a technique to solve the problem, there is proposed, for example, a pneumatic tire having a tread provided with circumferentially extending grooves and in the crosswise direction of the tire with laterally extending grooves with an incline to the equatorial plane of the tire, where notches are formed roughly parallel to and adjacent to the laterally extending grooves. (Japanese Patent Application Laid-Open Publication No. 38412/1991).
However, such a pneumatic tire is not effective enough for preventing noise because both the laterally extending grooves and the notches come into contact with the ground and the sidewalls of both the laterally extending grooves and the notches are simultaneously vibrated. That is, the pneumatic tire disclosed in Japanese Patent Application Laid-Open Publication No 38412/1991 has not succeeded in solving the above problem and the problem to be solved remains.
A pneumatic radial tire in which the upper edges of the sidewalls of a circumferentially extending groove are made with slopes with an incline to the equatorial plane of the tire is also proposed in e.g., Japanese Patent Application Laid-Open Publication No. 117414/1995. However, this prior art pneumatic radial tire cannot relieve lateral vibration of its circumferentially extending grooves and thus cannot satisfactorily reduce air-column-tube resonance sounds. Hence, the pneumatic tire disclosed in Japanese Patent Application Laid-Open Publication No. 117414/1995 has not succeeded in solving the above problem.
There is further proposed a pneumatic radial tire having a tread provided with a rib and with a circumferentially extending wide groove, said rib having circumferentially extending thin grooves formed outside said circumferentially extending wide groove, wherein the edge zone between the circumferentially extending wide and thin grooves is lower than the rib (U.S. Pat. No. 4,936,363).
However, the pneumatic radial tire proposed in U.S. Pat. No. 4,936,363 was structured as above to prevent the edge of the rib from undergoing wear, and said lower edge zone has projecting and reentrance portions on the sidewalls the circumferentially extending wide groove, that is, the cross-sectional shape of the edge zone is uneven, while the outside surface also has projecting and reentrance portions. Therefore, this prior art tire may be effective for preventing wear on the rib, but can not relieve the vibration in the crosswise direction of the circumferentially extending groove of the rolling tire. Therefore, this tire has not attained sufficient prevention of noise resulting from air-column-tube resonance sounds.